1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image, such as a laser printer, copying machine, and a facsimile, and more particularly to an image forming apparatus available for such areas where the commercial power supplies have different voltages.
2. Description of the Related Art
In a laser printer of the type in which characters and graphic images are recorded by using a latent image as formed on a photoreceptor, a latent image as formed is developed by toner, and a toner image formed is transferred onto paper. To fix the toner image by heat, an electric heater, for example, a heat roller, is usually used.
Electric components making up the image forming apparatus may be classified into two types, those components as driven by a DC power source, such as transistors and ICs (integrated circuits), and those components as driven by an AC power source, such as the heater. Motors as a power drive source, and a ventilation fan, that have been driven by an AC power source, are frequently driven by a DC power source in recent days. This arises from the facts that the electric components can be controlled with high precision, and that the electric components with the same characteristics can be used in both an area where the frequency of a commercial power supply is 50 Hz, and another area where the frequency of a commercial power supply is 60 Hz.
To drive the heater and some specific electric components in the image forming apparatus, some image forming apparatuses still use the AC power supply because they consume relatively large power and they are controllable by the AC.
FIG. 13 schematically illustrates a circuit arrangement of a fusing unit that has been used in the image forming apparatus, such as a laser printer. As shown, a commercial power supply 1 is connected through a fuse 2 to a fusing heater 4. The heater 4, which is a filament-like heat generating element, is usually contained in a narrow quartz tube.
FIG. 14 shows a mechanical structure of the fusing unit. The heater 4 is fixed to support plates 5.sub.2 by a set of lamp supports 5.sub.1. The support plates 5.sub.2 support collars 6.sub.1 of a heat roller 6 with the aid of bearings 5.sub.3. The heat roller 6 is a metal tube, which is coated with heat-resistant resin and has a gear 6.sub.2, at one end thereof. The gear is in mesh with another gear (not shown). The heat roller receives a turning force through the gear, and is turned by the force. The fuse 2 is in a slight contact with the surface of the heat roller 6 to monitor its surface temperature. One of the power supply lines, which connected to a connector 7, is directly coupled at one end (left end as viewed in the drawing) of the heater 4. The other power supply line is coupled at the other end (right end) of the heater 4, by way of the fuse 2. When the heat roller 6 is accidentally over heated, the fuse 2 is blown to stop the current feed to the heater 4.
A pressure roller 8 being heated is in press contact with the heat roller 6, to provide a nip of a predetermined width. Recorded paper passes through the nip, so that a toner image formed thereon is thermally fixed on the paper.
At this day, industrial products are distributed throughout the world, through import and export. Particularly in the case of a small or portable product, it is frequently carried by its owner, and used in several countries. In this case, the different voltages of the commercial power supplies in the countries become problematic. For example, the power supply voltage in Japan is 100V. In North America, it is 115V or 120V in most of the States. In Middle and Near East, Africa, and Europe, the power supply systems of 220V to 240V are dominantly employed. In adjacent countries or in different regions in the same country, the power supply voltages are often different.
The battery-driven electric products do not require an AC power source. Accordingly, no measure must be taken for the voltage difference of the commercial power supplies of every region or country. As for the DC electric components, such as ICs and DC motors, a power supply forms both voltages 24V and 5V in the stage of converting from an AC power supply into a DC power supply. Accordingly, also in those electric components, there is no need of taking any measure for the power supply voltage difference.
In the case of the fusing unit shown in FIGS. 13 and 14 using the electric components that are directly driven by the AC power supply, the electric components cannot be used from one region to another region where the power supply voltage is greatly different. For example, if a heater specified to be operable at 100V is used in a region where the power supply voltage is 200V, more current flows through the heater, which may damage the heater or burn related circuit parts. On the other hand, if a heater specified to be operable at 200V is used in a region where the power supply voltage is 100V, the heater is insufficiently heated, so that the fusing unit containing the heater fails to fix the toner image or it takes a long time until the satisfactory fuse fix becomes possible. Accordingly, such a use of the heater is unpractical.
For the above reasons, the electric components such as the heater are manufactured for each commercial power source voltage. Those components are selectively assembled into the copying machines according to regions or countries where the machines are to be used. Accordingly, in manufacturing the image forming apparatuses to be exported to many countries, the electric components operable by the DC power are available for all of the apparatuses. However, the electric components operable by the AC power must be specified according to the electric power supply systems employed by the countries. Many types of electric components, which comply with the specifications of the importing countries, must be manufactured and stored. This increases the cost to manufacture the components, and requires intricate management of storing many types of components. The heater, as shown in FIG. 14, is usually covered with a quartz tube, for example, in order to ensure a high temperature. The quartz tube is fragile, and is easily broken when it is carelessly touched by laymen not accustomed to handling it. However, it is not practical to have servicemen carry and replace all the quartz tubes necessary for the different power supply voltages used.